In a large microwave-safe bowl, combine Corn Chex and Rice Chex. Place butter in a small microwave-safe bowl. Microwave on high for 20-25 seconds or until just melted. Whisk in popcorn seasoning until smooth. Pour over cereal; toss to coat.
Microwave, uncovered, on high for 2-3 minutes, stirring after each minute. Immediately spread onto waxed paper; cool completely. Stir in popcorn and candies. Store in an airtight container.